Materialization (ToList, ToDictionary) & perf notes
Turn deferred queries into concrete collections with ToList/ToArray/ToDictionary; avoid repeated enumeration and know key-uniqueness rules.
Materialization (ToList, ToDictionary) & perf notes is a free C# Academy lesson on CoddyKit — lesson 2 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why materialize
Deferred queries run each time you enumerate.
- Materialize to take a snapshot
- Avoid multiple re-runs of expensive queries
- Choose List, Array, or Dictionary for the job
ToList / ToArray
ToList/ToArray run the query once and store results; later source changes are not reflected.
using System;
using System.Collections.Generic;
using System.Linq;
public class Program
{
public static void Main(string[] args)
{
List<int> data = new List<int>(new int[] { 1, 2, 3, 4 });
var evensQuery = data.Where(x => x % 2 == 0);
int[] evensArray = evensQuery.ToArray(); // snapshot now
List<int> evensList = evensQuery.ToList(); // snapshot now
data.Add(6); // changes after materialization
Console.WriteLine("Array:");
foreach (int x in evensArray) Console.WriteLine(x); // 2,4
Console.WriteLine("List:");
foreach (int x in evensList) Console.WriteLine(x); // 2,4
}
}
ToDictionary basics
ToDictionary materializes to a Dictionary<K,V>. Keys must be unique, otherwise it throws.
using System;
using System.Collections.Generic;
using System.Linq;
public sealed class City
{
public string Name;
public int Population;
public City(string name, int pop){ Name = name; Population = pop; }
}
public class Program
{
public static void Main(string[] args)
{
var cities = new City[]
{
new City("Oslo", 634000),
new City("Rome", 2873000),
new City("Riga", 605000)
};
Dictionary<string, int> byName = cities
.ToDictionary(c => c.Name, c => c.Population);
Console.WriteLine(byName["Rome"]); // 2873000
}
}
Duplicates: ToLookup/GroupBy
When keys can repeat, prefer ToLookup (multi-value dictionary) or GroupBy instead of ToDictionary.
using System;
using System.Collections.Generic;
using System.Linq;
public sealed class Item { public string Category; public string Name; public Item(string c, string n){ Category=c; Name=n; } }
public class Program
{
public static void Main(string[] args)
{
var items = new Item[]
{
new Item("Food","Apple"),
new Item("Food","Bread"),
new Item("Tool","Hammer"),
new Item("Tool","Wrench")
};
// If keys may repeat, use ToLookup (multi-value) or GroupBy
var lookup = items.ToLookup(i => i.Category, i => i.Name);
foreach (var g in lookup)
{
Console.WriteLine(g.Key + ":");
foreach (var name in g) Console.WriteLine(" - " + name);
}
}
}
Avoid multiple enumeration
Each enumeration can re-run work. Materialize once (e.g., ToList) when you will iterate multiple times.
using System;
using System.Collections.Generic;
using System.Linq;
public static class Expensive
{
public static IEnumerable<int> Numbers()
{
// Simulate expensive enumeration (prints when enumerated)
for (int i = 1; i <= 3; i++)
{
Console.WriteLine("Produce " + i);
yield return i;
}
}
}
public class Program
{
public static void Main(string[] args)
{
var query = Expensive.Numbers().Select(x => x * x);
Console.WriteLine("First pass:");
foreach (int x in query) Console.WriteLine(x);
Console.WriteLine("Second pass (re-enumerates work):");
foreach (int x in query) Console.WriteLine(x);
Console.WriteLine("Cache with ToList:");
List<int> cached = query.ToList(); // runs once
foreach (int x in cached) Console.WriteLine(x); // no extra production logs
}
}
Materialization checklist
Checklist:
- Use ToList/ToArray to snapshot.
- Use ToDictionary when keys are unique; else ToLookup/GroupBy.
- Avoid multiple enumeration of expensive sources; cache once.
- Materialize near boundaries (I/O, DB calls) for predictable behavior.
ToDictionary requirement
Recap
Recap: Materialize with ToList/ToArray for snapshots, ToDictionary for fast lookup (unique keys), and cache to avoid re-running expensive queries.
Frequently asked questions
Is the “Materialization (ToList, ToDictionary) & perf notes” lesson free?
Yes — the full text of “Materialization (ToList, ToDictionary) & perf notes” is free to read here on the web, and the C# Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO.
What will I learn in “Materialization (ToList, ToDictionary) & perf notes”?
Turn deferred queries into concrete collections with ToList/ToArray/ToDictionary; avoid repeated enumeration and know key-uniqueness rules. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start C# Academy?
No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 3, so you can start here or from the beginning and move at your own pace.
How long does the “Materialization (ToList, ToDictionary) & perf notes” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this C# Academy lesson?
Yes. Every C# Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- GroupBy, Join, Aggregate, projections (anonymous types)
- Materialization (ToList, ToDictionary) & perf notes
- Composability tips